The purpose of this project is to elucidate the mechanisms involved in recovery from vestibular injury (vestibular compensation) in primates. Experiments focus on the physiological processes underlying recovery of the vestibulo-ocular reflex following unilateral labyrinthectomy or plugging of the semicircular canals. The study will consider compensation of vestibular response during both passive and active head movements. For passive head movements, vestibular compensation has been found to be incomplete. The neuronal substrate for compensation of reflexes to both translational and rotational movement will be investigated using extracellular single unit recordings from the rostral bilateral vestibular nuclei both before and after peripheral vestibular lesion. Hypotheses concerning the central mechanisms of compensation of dynamic vestibular reflexes will be directly tested. These hypotheses include a role for plasticity in the vestibular commissural system and that permanent deficits in the otolith-ocular system are a result of permanent reorientation of the linear acceleration sensitivities of central neurons that encode translational and gravity information. For active head movements, preliminary studies have investigated the role played by volition in compensatory ocular responses during head movement by analyzing unrestrained eye and head saccadic movements (head-free gaze shifts) after labyrinthine lesion. These studies suggested that a subject is able to utilize copies of neural signals that encode head movements and/or cervical or visual feedback to facilitate behavioral compensation during natural, unrestrained head movements. This study will also test the hypothesis that copies of neural signals encoding volitional head movements are used during those head movements to modify the response of these central vestibular neurons, facilitating the stabilization of gaze during rotational movements in compensated animals. This research is an outgrowth of the P.l.'s interest in vestibular compensation developed from his previous research experience and from his work as an otolaryngologist treating patients complaining of dizziness.